Multilayer films have been utilized for the packaging of "oxygen-sensitive products", such as lettuce, i.e., products which exhibit lower shelf life in the presence of either too much oxygen in the package, or too little oxygen in the package. In such multilayer films used for packaging oxygen-sensitive products, the O.sub.2 -transmission rate, and even the carbon dioxide transmission rate, are of primary importance, especially in the packaging of such oxygen-sensitive products as vegetables, fruits, and cheese. For example, in the packaging of precut lettuce, the presence of too much oxygen in the package results in enzymatic browning of cut surfaces, known as pink ribbing. On the other hand, if the concentration of oxygen in the package is too low, the lettuce tends to spoil due to anaerobiosis.
Cut lettuce is frequently packaged using vertical form fill and seal (VFFS) equipment. Produce packaging films used on VFFS equipment have been composed from a variety of materials, including polyethylene, ethylene/vinyl acetate copolymer, ethylene/methyl acrylate copolymer, blends of polyethylene and ethylene/vinyl acetate copolymer, and polypropylene. Such films are typically multilayer films.
Furthermore, different products, such as lettuce and broccoli, exhibit maximum shelf-life when packaged in films having O.sub.2 -transmission rates substantially different from one another. There are many O.sub.2 -sensitive products having a shelf-life which is dependent upon the O.sub.2 -transmission rate of the package within which they are contained, and the optimum O.sub.2 -transmission rates for the packaging of these products varies widely. As a result, it is desirable to provide a process for producing a variety of multilayer films having differing O.sub.2 -transmission rates.
Gas transmission rates for vegetable packaging films have traditionally been tailored to a desired level by varying the overall thickness of the multilayer film. That is, in order to achieve a desired, relatively high O.sub.2 -transmission rate, a thinner film is produced. Such downgauging is often done at the expense of film strength and abuse resistance. Conversely, film structures which are abuse-resistant and machinable generally lack the desired level of gas permeability and sealing properties required for such applications as use in vertical form fill and seal equipment. It is therefore desirable to provide a film which combines abuse resistance with a relatively high O.sub.2 -transmission rate.
Moreover, it would be desirable to utilize a film structure and composition which permits structural and compositional changes so that a set of multilayer films of a substantially singular thickness exhibit a relatively wide spectrum of O.sub.2 -transmission rates and CO.sub.2 -transmission rates. In this manner, overall film physical properties can be kept substantially constant, while at the same time films can be tailored to the optimal O.sub.2 -transmission rates and the optimal carbon dioxide transmission rates of a variety of O.sub.2 -sensitive products.
Furthermore, it would be advantageous to provide such a film with a composition and structure which has desirable sealing characteristics, i.e., does not exhibit burn through, does not pucker at the seal, exhibits good hot tack, and seals quickly and at relatively low temperature, while simultaneously permitting optimization of oxygen and carbon dioxide transmission rates, via the ability to vary the structure and composition of the multilayer film, without substantially increasing or substantially decreasing the overall thickness of the multilayer film.
It is also important for multilayer films used in vertical form fill and seal equipment to have surface layers sealable with hot bar and impulse type sealing systems. Other desirable attributes are abuse resistance, clarity, and a modulus sufficient that the film has good machinability on vertical form fill and seal equipment.
Prior art films useful for the packaging of O.sub.2 -sensitive products have been found to lack the combination of desirable OTR for maximum shelf life, machinability properties to run on vertical form fill and seal equipment, high hot tack strength, as well as optical properties such as high gloss and low haze.
Films used on vertical form fill and seal (VFFS) equipment are subjected to at least two different kinds of seals being produced during the packaging of the product, i.e., a longitudinal or vertical seal, and horizontal bottom and top seals. Accordingly, in the construction of such packages, characteristics such as the temperature required to form the seal, adequate heat-resistance to prevent burn through, and a very high percentage of continuous (nondefective) seals, are all important in assessing the performance of any given film for the packaging of O.sub.2 -sensitive products.
Fogging of the package is also a common problem in the packaging of produce and other food products. It would be desirable to provide a package comprising a film which resists fogging, in order to provide the consumer with a clear view of the contents of the package, and in order to provide a more aesthetically appealing package, especially in retail applications where product presentation is important. However, surface active agents which are effective antifog agents tend to interfere with ink adhesion to the film. This detrimental effect occurs because the antifog agent blooms to the outside surface of the package and interferes with the adhesion of the ink to the film. This detriment is significant for packages designed for consumer end use, as the consumer does not find such a package to be appealing if the ink is smeared or if the ink comes off onto other articles or the consumer. Thus, it would be especially desirable to provide a film having a desired O.sub.2 -transmission rate suitable for the packaging of various products, wherein the film further comprises an antifog agent on an outer film surface which forms the inside surface of the package, while also providing adequate adhesion for printing on an outer surface of the film, which outer surface serves as the outside surface of the package.